When I select the SFTP option to transfer files, and either the local or remote folder contains a large number of files, it takes FOREVER for AbsoluteTelnet to populate the list. In some cases, it's minutes, not seconds, and freezes the user interface. This feature is basically not usable for me because of this.

I'm not sure what language AT is written in, but if you're using a standard Windows list object, sending a WM_SETREDRAW message to the control before (with FALSE for wParam) and after (with TRUE for wParam) building the file listing can reduce the amount of time by several orders of magnitude. At least the portion of building the list that occurs in Windows APIs anyway.